1. Field of the Invention
This invention relates to a single-part annular reinforcement (annular support) for piston ring grooves of light-metal pistons, with cylindrical borings which are provided inside the annular support and which, when the support is built into the piston, are filled with piston material.
The main problem arising in the use of annular supports for the reinforcement of piston ring grooves is that of firmly securing them inside the material of the piston.
2. Description of the Prior Art
Methods of securing the annular support in the piston material are fundamentally subdivided into chemical and mechanical connections and a combination of the two. To obtain a satisfactory mechanical connection, a number of different external shapes for the annular support are already known. For a firm mechanical connection it is important that the external shape of the annnular support should be such that the piston material immediately adjacent thereto should be able to engage apertures of the undercut type. An external shape of this kind for the annular support is obtained, for example, with a fissured cast skin with numerous irregularly undercut cavities in accordance with German Pat. No. 1,224,104. The same purpose is fulfilled bu dovetailed securing devices on the outer surfaces of an annular support, such as shown in Austrian Pat. No. 247,072. In brief, the securing system must be such that the annular support engages undercuts in the piston material with a claw-type engagement in all directions.
This claw-type engagement is absent from the known annular supports with borings serving to accommodate piston material. An example is provided by the annular support system covered by German Pat. No. 1,165,934. For in this system the borings are parallel to the longitudinal axis of the piston and therefore not capable of securing the annular support against displacements in the said direction. Similar considerations apply to the conical borings provided in an annular support in accordance with German Pat. No. 1,038,833 and running parallel to the longitudinal axis of the piston. Apart from the fact that this is in any case merely a sheet metal insert for a flank of a piston ring groove, conical borings paraxial to the axis of the piston will likewise only constitute a form-closed securing device for the annular support to one side in the said direction, i.e., towards the side having the smaller boring diameter. In the opposite direction, i.e., towards the side with the greater boring diameter, the said supporting action is naturally absent. Yet it is precisely in this latter direction that a mechanical supporting action such as obtained by undercuts in the dovetailed system is particularly important. The individual flanks of a single-part annular support thus cannnot be given a mechanical claw-like engagement in the piston material by providing conical borings, in which for reasons connected with the technique of manufacture the greater boring diameter must naturally always be situated on the outer surface of the annular support. This is doubtless also one of the reasons why the reinforcement inserts provided by German Pat. No. 1,038,833 for the annular grooves are characterized not only by borings but also by securing devices of the dovetail type.
One of the results of the difficulties experienced in producing recesses such as conical borings which would act like undercuts and be simple to work into the material was that in the case of the annular support suggested in U.S. Pat. No. 2,996,341 two separate elements were required for the reinforcement of the upper surface and the lower surface of the groove. It is only in the base of the groove that these elements rest loosely against each other. In practice, therefore, this system fails to provide a means of preventing displacement in all directions. For here, just as in the case of the conical borings of the annular support covered by German Pat. No. 1,038,833, the reinforcements of the flanks of the groove are not secured in the axial direction.